Vasovagal syncope is a condition marked by a sudden drop in heart rate and blood pressure, resulting in fainting. It is not only unpleasant for a patient, but potentially dangerous, as fainting may lead to injuries from falls and increase the risk of motor vehicle accidents. The condition affects many thousands of patients, and particularly, the elderly. Many such patients present at emergency rooms with a wide variety of injuries due to falling.
A cardiac pacing system is disclosed in U.S. Pat. No. 5,284,491 that is specifically adapted to treat patients suffering from vasovagal syncope. In particular, the pacemaker detects when the patient's heart rate drops below a lower “hysteresis” rate and determines whether the average rate of decrease in the patient's heart rate, over a defined number of heartbeats or a defined time interval prior to reaching the “hysteresis” rate, is greater than a preset value. If so, the pacing rate is set equal to the “hysteresis” rate and thereafter increased to an “intermediate” rate substantially higher than the “hysteresis” rate. The pacing rate remains at the “intermediate” rate for a preset time period and thereafter gradually declines to a lower pacing rate.
Commonly assigned U.S. Pat. No. 5,501,701 discloses an improved pacing system for the treatment of patients with vasovagal syncope that differs from the prior pacing system disclosed in the Sutton, '491 patent in that the methods of detection of an episode of vasovagal syncope and the intervention therapy delivered in response are refined. A persistent rate above a first threshold rate is required to initiate the rate drop detection function rather than detecting a rapid rate drop thereby preventing single rapid heartbeats from triggering detection of a rapid rate drop. A persistent or stable heart rate (e.g., x of y beats less than the drop rate) is required following detection of a rapid rate drop from above the first threshold rate to a rate below a second threshold rate or “drop rate” before the pacing rate is changed. Criteria for exiting the intervention therapy and for exiting the detection process due to sensed spontaneous depolarizations are also provided.
Commonly assigned U.S. Pat. Nos. 5,540,728 and 5,676,686 disclose further improved pacing systems for the treatment of patients with vasovagal syncope wherein the detection of rate drop employs a process for defining the highest persistent rate over a period of time preceding the fall of the heart rate below the drop rate. The heart rate is monitored over a series of time intervals, with the fastest two-beat sequence in each time interval identified. The rate of the slower of the two beats in the identified sequence is stored as a “top rate”, and the fastest of the “top rates” is identified as the highest persistent rate. The measured rate drop is then taken as the difference between the fastest such “top rate” and the heart rate following a drop below the drop rate. This process prevents short intervals as might result from premature depolarizations of the atrium or ventricle from erroneously triggering pacing at an increased rate.
In a further preferred embodiment, an alternative method of detecting vasovagal syncope and triggering an increased pacing rate is provided. The pacemaker in this embodiment keeps count of successive paced beats, and triggers an increased pacing rate in response to an extended series of paced beats at the base pacing rate.
Moreover, the vasovagal syncope detection function can be disabled during a defined sleep period, so that normal rate drops associated with sleep do not result in inappropriate triggering of pacing at an increased rate that could unnecessarily disturb the patient's sleep.
Further U.S. Pat. No. 6,049,735 incorporates a detection algorithm in which a precipitous rate drop from a pre-existing average intrinsic heart rate value to a lower rate limit is sensed. If the patient is awake and the intrinsic rate remains below the lower rate limit for a predetermined number of beats or interval, the patient's heart is paced at a rate that is a programmed differential above the prior average intrinsic rate for a second programmed time interval. The pacing rate is then gradually decreased to the average heart rate existing prior to the sudden rate drop or to the lower rate limit.
The pacing system is preferably incorporated into a rate adaptive pacemaker having means for sensing a parameter proportional to hemodynamic demand of the patient along with provisions for the conventional lower rate limit and upper rate limit thresholds. A drop in heart rate from the computed average heart rate existing prior to the drop down to a preprogrammed lower sensor rate limit for a predetermined number of beats triggers delivery of pacing pulses at a predetermined differential rate in excess of the average heart rate value existing immediately prior to the drop in heart rate.
To avoid false positives, the implantable cardiac rhythm management device may also include a sensor for determining minute ventilation, allowing respiratory rate and tidal volume parameters to be derived. The therapy delivery means is not only responsive to the rate drop sensing means, but also to the respiratory parameters to inhibit application of therapy pacing pulses above the average heart rate when the respiratory parameters are below a prescribed threshold indicative of a patient during sleep.
An implantable drug infusion device is disclosed in U.S. Pat. No. 5,919,210 that detects and treats syncopal episodes by infusing a suitable drug, e.g. beta blockers, alone or in combination with drug therapies. An algorithm is disclosed for determining the presence of syncope based on physiological activity associated with the onset of a syncopal episode. The physiological activities detected and used by the device are selected from the group including changes in heart rate, heart rate variability, QT interval, PR interval, pressure, blood flow, vagal nerve activity, temperature, pH, AV conduction times, respiration rate, position, motion, and combinations thereof.
Use of certain of these “physiological activities” in an implantable medical device is also disclosed in U.S. Pat. No. 5,464,434 to provide a number of different controllable therapeutic functions including cardiac pacing, anti-tachycardia pacing, cardioversion, and defibrillation. The system of the '434 patent is directed toward detection of syncopal episodes associated with a malignant tachyarrhythmia warranting delivery of an anti-tachyarrhythmia therapy. Variations of one or more physiologic parameter of the patient indicative of the substantially instantaneous hemodynamic condition of the patient's heart are monitored. The physiologic parameter(s) sensed may be any of blood pressure, blood oxygen content, minute ventilation, central venous temperature, pulse rate, blood flow, physical activity, or other parameter for that purpose. The mean and standard deviation of a signal generated by the sensor over a predetermined time interval are calculated. The quotient of the standard deviation and the mean is also calculated in order to determine a sudden hemodynamic change such as a precipitous drop in cardiac output. An anti-tachyarrhythmia therapy, e.g. a defibrillating protocol, is then triggered.
Despite these improvements, there is a need to be able to detect the onset of a syncopal episode with greater speed and accuracy to provide a suitable pacing therapy to decrease the likelihood that the patient will faint.